Dynamic range or contrast ratio is the ratio of intensity of the highest luminance parts of a scene and the lowest luminance parts of a scene. For example, an image projected by a prior art video projection system may typically have a maximum dynamic range of 300:1. It is desired to have image display apparatus with higher dynamic range.
Image display apparatus with enhanced dynamic range are known. U.S. Pat. No. 5,978,142 describes a monochromatic image display apparatus comprising projector means, a light source, a first modulator, a second modulator and relay means, the first and second modulators both being such as to modulate light received from the light source, the first modulator modulating the light to produce an image, the relay means optically relaying the image to the second modulator, the second modulator being such as to further modulate the image to perform optical image enhancement whereby finite minimum brightness above true black is reduced, whereby contrast range of the projected image is extended, and whereby the optical image enhancement achieved is determined by a driving signal to the second modulator.
U.S. Pat. No. 6,985,272 applies the above technique to multi-colour image display apparatus and describes an image display apparatus comprising projector means, optical means for splitting incident light into red, green and blue light, a first modulator for modulating red light, a second modulator for modulating green light, a third modulator for modulating blue light, and optical means for recombining the modulated red light, green light and blue light. A fourth modulator is provided for modulating the combined red light, green light and blue light, and control means in the form of look-up tables for controlling the operation of the first, second, third and fourth modulators such that the dynamic range of the image display apparatus is enhanced.
It is known in the industry that the dynamic range of video images produced by light modulators such as LCD and others can also be extended by dynamically modulating the intensity of the backlight illumination system in response to the desired intensity of its video input. This intensity change may be essentially equal across the backlight of the entire display.
WO 2006/045585 describes a display with a backlight having a temporal modulation applied and a pixel addressable LCD in an optical path, the pixel addressable LCD being arranged to output each pixel of a frame as a temporal sequence of output values, different values of the sequence coinciding with different output levels of the modulated backlight. The apparent luminance or colour of the pixels can be made to take intermediate values between the gradations dictated by the stepsize corresponding to a least significant bit of the pixel addressable LCD, to enable more accurate reproduction of both colour and greyscale images. A convertor generates a temporal modulation of the pixels for the LCD according to a value of the pixels in an input signal, and synchronised to the temporal modulation of the backlight.
Display results when applying the above backlight modulation technique to multiple display channel applications are not satisfactory as disturbing artefacts between adjacent images are noticeable.